The cardiac cycle of the embryonic heart has a distinct atrial and ventricular systole and diastole. Cardiac output is affected by the reservoir, conduit, and pump functions of the atrium. Varying the magnitudes of various system parameters such as heart rate, atrial- ventricular coupling and the diastolic properties in the embryonic heart. The combined analysis of atrial and ventricular function is integral to our comprehension of normal and experimentally altered cardiovascular performance. Our long term aim is to understand the integrated cardiovascular function during morphogenesis of the cardiovascular system. This is fundamental in order to mathematically model atrial/ventricular coupling. We hypothesize that there is optimal hemodynamic and mechanical coupling of atrial and ventricular function in the embryonic cardiovascular system. Our experimental model is the stage 16 to 29 white Leghorn chick embryo during the transaction of ventricular geometry from a smooth-walled to a trabecular chamber. Our experiment methods are described in the Physiology Core, and include the measurements of servonull pressure, Doppler velocity and video imaging. Our specific aims are: Define atrial pump function, and the atrioventricular pressure gradient during normal cardiovascular development and during acute alterations in preload and calcium availability. Define the transfer of blood pressure flow between the embryonic atrium and ventricle during alterations in heart rate produces by thermal probe application, embryo hypothermia, and sinus venosus pacing. Define the mechanical and biochemical basis for diastolic relaxation and filling during normal cardiovascular development, and following acute alterations in preload, calcium availability, and ventricular growth acceleration produced by conotruncal banding and growth declaration produced by partial left atrial ligation. This project defines the functional interaction of the embryonic atrium and ventricle during a wide range of acute and chronic alterations in hemodynamic performance. The analysis of experimental results within the framework of a closed loop system provides crucial information of the regulation of cardiovascular function. This integrated analysis then becomes the foundation for the structural and functional analysis of genetically altered cardiovascular systems in experimental models, and ultimately, aids in defining the underlying etiologies of congenital cardiovascular malformations.